(1) Field of the Invention
The present invention relates to a transmission line monitoring system for monitoring the quality of a transmission line along which a plurality of repeaters are equipped.
In communication network systems wherein communication terminals and offices are connected by transmission lines, generally a plurality of repeaters are equipped along the transmission lines for retiming, reshaping, and amplifying signals transmitted through the transmission lines.
In the above communication network systems, sometimes trouble occurs in a transmission line due to: a degradation of an AGC (automatic gain control) circuit in a repeater; a damage by a serge current caused by a thunderbolt; a degradation of a cable due to aging; a break in a cable; a malfunction of a power feeding circuit, and the like.
The repeaters are located, for example, in manholes or on telephone poles, and are generally unmanned. Therefore, the transmission lines are usually monitored at a terminal station regarding an occurrence of trouble, and when trouble occurs in a transmission line between communication offices, it is necessary first to determine where the trouble has occurred.
(2) Description of the Related Art
In the prior art, the following systems are proposed to monitor a transmission line and to determine the location of trouble in a pair of transmission lines (a two-way transmission line).
The Japanese Unexamined Patent Publication No. 54-47508 discloses loop back systems providing relay switches in repeaters and providing band pass filters in repeaters.
In the loop back system using relay switches, first and second relay switches and a control circuit are provided in each repeater which is located along a pair of transmission lines (a two-way transmission line). The first and second relay switches are each located at the front and rear sides of each repeater, respectively.
When a direct current signal is first sent from a monitoring station through the transmission line to a nearest repeater, a first relay switch in the nearest repeater connects the transmission lines in a sending direction and in a receiving direction in the repeater, so that a loop back circuit is made at the front side of the repeater, for carrying out a loop back test not including the repeater circuit.
When the next direct current signal is sent from the monitoring station through the transmission line to the above nearest repeater after the above loop back test is carried out, a second relay switch in the repeater connects transmission lines in a sending direction and in a receiving direction in the repeater, so that a loop back circuit is made at the rear side of the repeater, for carrying out a loop back test including the repeater.
When the third direct current signal is sent from the monitoring station through the transmission line to the above nearest repeater after the above loop back tests are carried out, the control circuit sends the signal to the next repeater, and then, similar operations for the loop back tests are performed in the next repeater.
In the loop back system using band pass filters, a plurality of band pass filters each having a different center frequency are provided at the front and rear sides of all the repeaters located along a pair of transmission lines (a two-way transmission line). Each band pass filter is connected between transmission lines in a sending direction and in a receiving direction at its position.
The loop back test is carried out by sending signal having a frequency which is equal to the center frequency of a band pass filter located in an objective position.
However, in both the above loop back systems using relay switches and using band pass filters, if troubles occur in a plurality of locations in a transmission line, the locations of the trouble except the location nearest to the monitoring station, cannot be determined.
The Japanese Unexamined Patent Publication No. 64-53657 discloses a system for monitoring an error rate in an output of a repeater for one of a plurality of transmission lines. The plurality of transmission lines are equipped in parallel between communication terminals, and a repeating operation for each transmission line is carried out in an amplifier. The amplifiers are provided for respective transmission lines, and contained in a repeater unit.
In the above system, a monitoring unit is provided for each repeater, a monitoring line is equipped along the transmission lines, and communication for collecting information on the error rate in each repeater is carried out between each monitoring unit and a monitoring station through the monitoring line using FSK (frequency shift keying) signals.
In addition, pulse transformers are provided at both sides of each amplifier, and the pulse transformers at the output side of the amplifiers each have an additional winding to monitor an output of a corresponding amplifier during a normal operation.
One of the monitored outputs is selected by a switch, the (bipolar) output of the switch is transformed to a unipolar signal, and the unipolar signal is input into an error monitor circuit. The output of the error monitor circuit is encoded in a CPU to a three-bit coded signal which indicates an error rate. The above switch, the error monitor circuit, and the CPU are contained in the monitoring unit.
The monitoring station sends address information including an address of an objective repeater, and an identification number of an objective transmission line, through the monitoring line to the objective repeater. When the CPU detects an address of a corresponding repeater, the CPU sends the above code indicating the error rate together with its own address through the monitoring line to the monitoring station. The monitoring station collects the error rate information of all the repeaters by an polling operation.
However, the object of the above system for monitoring an error rate in an output of a repeater is mainly to obtain information on the degradation of each repeater, and only error rate information in each repeater can be obtained. Therefore, it is not clear where the cause of the error in the output of a repeater exists, for example, when a break in a transmission line occurs, no information is obtained from the repeaters beyond the break point because no signal is transmitted beyond the break point, and therefore the monitoring of error rates is meaningless.
FIG. 1 shows an outline of the construction of an example of a conventional two-way communication system having a plurality of repeaters along a two-way transmission line connecting terminal repeater units.
In FIG. 1, reference numerals 1 and 2 each denote a terminal repeater unit, 30, 30', and 30" each denote a repeater, 11, 11', 11", 12, 12', 12", and 50 each denote a regenerative repeating unit, 31, 32, 32', 32", 51, 51', 51", 34, 35, 36, 37, 37', 37", 52, 52', 52", and 39 each denote a pulse transformer, 53, 53', and 53" each denote a Zener diode, 54, 54', and 54" each denote a resistor, 55, 55', and 55" each denote a diode, and 40 denotes a power feeder circuit.
In the communication system shown in FIG. 1, data signals are transmitted between the terminal repeater units 1 and 2 through the transmission lines 4 and 4', and a plurality of repeaters 30, 30', and 30" are provided along the transmission line 4. The pulse transformers are provided at input and output sides of each of the terminal repeater units 1 and 2, and the repeaters 30, 30', and 30".
The power feeder circuit 40 is provided in one of the terminal repeater units 1, and an electric power is supplied in the form of a constant electric current from the power feeder circuit 40 to each repeater through the transmission lines 4 and 4' by connecting the output terminal of the power feeder circuit 40 to both of the secondary winding of the pulse transformer 31 and the primary winding of the pulse transformer 39, at the output side of the terminal repeater unit 1. The power is supplied to two regenerative repeating units for two-way transmission in each repeater as a terminal voltage of a corresponding Zener diode 53, 53', or 53", where the one terminal of each Zener diode is connected to the primary winding of the pulse transformer 32, 32', or 32", and the other terminal of each Zener diode is connected to the secondary winding of a corresponding pulse transformer 51, 51', or 51". The circuit for feeding the power to the repeaters 30, 30', and 30" is completed by further connecting a primary winding of the pulse transformer 34 to a primary winding of the pulse transformer 36, and connecting a primary winding of a pulse transformer 37, 37', or 37" in the input side to a secondary winding of a corresponding pulse transformer 52, 52', or 52" in the output side of each repeater 30, 30', or 30".
The terminal repeater unit 1 (the power feeder circuit 40 in the terminal repeater unit) functions to output an alarm signal when a direct current does not flow from the power feeding circuit through the transmission lines 4 and 4'.
The resistor 54, 54', or 54" and a corresponding diode 55, 55', or 55" are connected in series between the primary winding of a pulse transformer 32, 32', or 32" and the secondary winding of a pulse transformer 52, 52', or 52" in each repeater. The diode 55, 55', or 55" in each repeater is connected in the direction opposite to the voltage supplied by the power feeder circuit 40.
The resistors 54, 54', and 54" and the diodes 55, 55', and 55" in the repeaters 30, 30', and 30" are connected as above for another conventional method for monitoring the transmission line as explained below with reference to FIG. 2.
FIG. 2 shows a conventional transmission line monitoring system for monitoring the transmission lines 4 and 4' in the communication system of FIG. 1.
In FIG. 2, the terminal repeater unit 1 in the construction of FIG. 1 is replaced with a testing unit 100 for determining a position of a break in the transmission lines 4 and 4'.
In the testing unit 100, reference numeral 56 denotes a voltage source, 57 denotes an ampere meter, 58 denotes a display device using a light emitting device (LED), and 59 and 60 each denote a pulse transformers.
The pulse transformers 59 and 60 are provided at the output side of the testing unit 100, a secondary winding of the pulse transformer 59 is connected to the transmission line 4, and a primary winding of the pulse transformer 60 is connected to the transmission line 4'.
The voltage source 56 generates a constant voltage. One terminal of the voltage source 56 is connected to the secondary winding of the pulse transformer 59, and the other terminal of the voltage source 56 is connected to one terminal of the ampere meter 57. The other terminal of the ampere meter 57 is connected to the primary winding of the pulse transformer 60. The direction of the connection of the voltage source 56 is opposite to the power feeder circuit 40, and therefore, the current from the voltage source 56 can pass through the series connections of the resistors 54, 54', and 54" and the corresponding diodes 55, 55', and 55" in the repeaters 30, 30', and 30", in parallel with the aforementioned current path through the transmission lines 4 and 4' and the Zener diodes 53, 53', and 53".
When no break occurs in the transmission lines 4 and 4', all the paths by the above series connections of the resistors 54, 54', and 54" and the corresponding diodes 55, 55', and 55" contribute to the total current value detected in the ampere meter 57, and when a break occurs in a position of the transmission lines 4 and 4', only the series connections of the resistors and the diodes located in the side of the terminal repeater unit 1 from the position of the break, can contribute to the total current. Therefore, the position of the break is determined by the total current value which is detected in the ampere meter 57, and is displayed on the display device 58.
In practice, when the communication system is installed, repeaters are connected one by one in the direction from the terminal repeater unit 1 to the other terminal repeater unit 2, and at the same time the current values are measured in respective conditions of the numbers of the connected repeaters.
In a monitoring operation, the testing unit 100 is connected in the place of the terminal repeater unit 1 as shown in FIG. 2, and the current value is detected by the ampere meter 57 and the display device 58. The detected value is compared with the above values measured in advance, and thus the section of the transmission lines 4 and 4' wherein a break occurs, is determined.
However, in the above monitoring system shown in FIGS. 1 and 2, similar to the aforementioned loop back systems using relay switches and using band pass filters, if troubles occur in a plurality of locations in a transmission line, the locations of the trouble except the location nearest to the monitoring station, and which one of a pair of transmission lines constituting a two-way transmission line has trouble, cannot be determined.